The futex_wait mechanism of current 'glibc':http://www.gnu.org/software/libc/ with enabled 'NPTL':http://en.wikipedia.org/wiki/Nptl can cause a deadlock, when a sublet uses e.g. 'system':http://www.ruby-doc.org/core/classes/Kernel.html#M001441 periodically. subtle will freeze then and 'strace':http://en.wikipedia.org/wiki/Strace just displays FUTEX_WAIT in that case.

Sublets that are known to cause this kind of problems are: pacman, storage